This invention relates to a substrate well adapted for fabrication of nitride semiconductor elements typically including high-electron-mobility transistors (HEMTs) and metal-semiconductor-field-effect transistors (MESFETs), to a method of making the semiconductor substrate, and to a semiconductor element including the substrate.
The substrate for use in fabrication of nitride semiconductor elements has a baseplate, or substrate proper, which is usually made from such familiar substances as sapphire, silicon carbide, or silicon. A nitride semiconductor layer or layers for the desired elements are grown epitaxially on the substrate proper. Being less expensive, silicon is preferable to sapphire and silicon carbide, as disclosed in Japanese Unexamined Patent Publication No. 2003-59948. Silicon is not, however, wholly satisfactory as a substrate material: There is an inconveniently great difference in linear expansion coefficient between the silicon substrate proper and the nitride semiconductor layer or layers. The nitride semiconductor layer or layers had been easy to be stressed as a result, with the consequent development of cracks and transitions.
In order to remedy this difficulty, the above cited patent application proposed an interposition of a multilayered buffer region between the silicon substrate proper and the nitride semiconductor layer or layers, the latter being grown epitaxially on the interposed buffer region. The multilayered buffer region favorably performed the purposes for which it was intended, mitigating the stresses and reducing cracks and transitions in the nitride semiconductor layers.
It has later proved, however, that the multilayered buffer region possessed its own shortcomings. Let us consider for example the case where the buffer region takes the form of several alternations of a relatively thin layer of aluminum nitride (AlN) and a thicker layer of gallium nitride (GaN). The GaN layers are higher in lattice constant than the AlN layers. As a consequence, if the heterostructure was distorted without lattice loosening, tensile stress would be applied to the overlying AlN layers, with the consequent creation of polarization charges at the heterojunctions. Two-dimensional electron gas layers would then be created in the GaN layers, as will be later explained in more detail with reference to the attached drawings.
The two-dimensional electron gas layers would make the GaN layers extremely low in resistance in their own planes. Thus the prior art multilayered buffer region provided a path of leakage current for the semiconductor elements created thereon and so actually invited an increase in the amount of leakage current.
For fabrication of desired semiconductor elements on the substrate, there is formed on the multilayered buffer region a main semiconductor region for such elements, in addition to a source electrode, drain electrode, and gate electrode for each element. The main semiconductor region is a lamination of what are known to the specialists as an electron transit layer and an electron supply layer in the case of HEMTs. The low resistance of the buffer region resulted in a flow of leakage current along the path sequentially comprising the source electrode, electron supply layer, electron transit layer, buffer region, electron transit layer, electron supply layer, and drain electrode, when a control signal is impressed to the gate of the HEMT to cause nonconduction therethrough. This current leakage is altogether undesirable and should be reduced to a minimum, not only in HEMTs but in MESFETs and other semiconductor elements of comparable design as well.
Hereinafter in this specification the term “substrate proper” will be used to refer to the baseplate of silicon or the like. The term “substrate system” will then refer to the combination of the baseplate, the multilayered buffer region thereon, and the main semiconductor region of one or more layers. The substrate system might also be termed a semiconductor body.